1. Field of the Invention
The present invention relates to a color image forming apparatus that electrophotographically forms a color image, and a process cartridge for the color image forming apparatus.
2. Description of the Related Art
Tandem-type color image forming apparatuses have units each of which adjusts a timing of starting image formation for each color. Japanese Patent Application Laid-Open No. 2003-255656 discloses a technology for a tandem-type color image forming apparatus that has delay adjustment functions each of which is discretely provided for one of the colors in color images and controls a timing of starting image formation.
An image forming apparatus is a device that forms an electrostatic image as an output to a transfer paper that is a recording medium. Functions required for the image forming apparatus are high resolution, high speed, and high density. The high resolution and the high speed are also required for monochrome image forming apparatuses. However, recently available color image forming apparatuses require higher resolution than ever before. The monochrome image forming apparatus forms a monochrome image on a specified position of a transfer paper. However, the color image forming apparatus needs to form images of a plurality of colors, four colors in general (black, magenta, cyan, and yellow) on a specified position of a transfer paper. Therefore, misregistrations between the images lead to color shifts, causing a degradation of the image.
As an electrophotographic image forming apparatus, a system for generating an operation start signal is known. The system generates the operation start signal based on a detection signal by a registration sensor that detects a position of a transfer paper. In this case, a registration position in a sub-scanning direction is adjusted during a time from generating the operation start signal to starting image formation of the colors. More specifically, such a tandem-type color image forming apparatus as follows is known. The tandem-type color image forming apparatus includes image forming units arranged along a transfer belt called an intermediate transfer belt. Each of the image forming units is provided for each color and includes a photosensitive element. The image forming units form the images of the colors, and the images formed are superposedly on the same position of the intermediate transfer belt. The images superposed are transferred to the transfer paper by a transfer unit.
In such a tandem-type color image forming apparatus, timings of starting image formation for the colors differ from one another. This is because it is necessary to superpose the images of the colors from the image forming units for the colors on one another on the same position of the intermediate transfer belt. Therefore, distances from the registration sensor to respective positions of image formation for the colors are different from one another. This type of color image forming apparatus therefore includes units, each of which adjusts timing for one of the colors until relevant image formation is started. For example, Japanese Patent Application Laid-Open No. 2003-255656 discloses the technology in which the tandem-type color image forming apparatus has the delay adjustment functions provided discretely for each of the colors of a color image and controls each timing of starting image formation.
It is also necessary to control registration between the colors in the sub-scanning direction (hereinafter, “sub-scanning registration”) according to each color registration. Therefore, generally, the registration is controlled based on a synchronization detection signal that is a reference timing signal for starting to write for each color in a main scanning direction.
However, the synchronization detection signals for the colors are detected at different timings in each color because the positions of image heights are different. Furthermore, the operation start signal generated based on the detection by the registration sensor is an asynchronous signal with respect to the synchronization detection signal for each color. Therefore, displacement for a line period occurs between image formations for the colors according to the operation start signal and the timing of the synchronization detection signal for each color, which results in occurrence of color shift for each color on the registration position in the sub-scanning direction, which causes occurrence of the color shift by one line period at maximum.
The technology disclosed in Japanese Patent Application Laid-Open No. 2003-255656 describes that the tandem-type color image forming apparatus based on an one-beam optical system for each color compares phases of the operation start signal and the synchronization detection signal for each color with a line period, adjusts an imaging timing in the sub-scanning direction for each color one line by one line, and prevents displacement between registration positions by one line in the sub-scanning direction.
In the above technology, however, it is necessary to provide a unit (measuring unit) for measuring an elapsed time and a unit (operating unit) for obtaining a predetermined difference. Therefore, it is desired to suppress displacement between pixels in the sub-scanning direction without using these units.
In the above technology, comparison and operation are performed based on a cycle T of the synchronization detection signal, but the cycle of the synchronization detection signal fluctuates depending on rotational speed variations or mirror surface precision of a rotating polygon mirror. In such a case, if the timing of an input of the operation start signal is synchronized to the synchronization detection signal, it is disadvantage that a determining unit causes timings of starting image formation of write controllers to be different so that each of the write controllers differently controls optical writing to form an electrostatic latent image. By causing reference time points of all the write controllers to be a single point based on only timings of inputting the synchronization detection signal of the write controller and of inputting the operation start signal, it is desired to prevent occurrence of nonuniform control in the write controllers.
Furthermore, the timings of starting image formation are made to be delayed line by line by a writing unit. When the write controller makes an image signal delayed line by line, it is necessary to provide a storage device (memory or so) capable of temporarily storing image data for one line. By causing a timing of making a transmission request for an image signal to be delayed but not by causing an image signal to be delayed in the write controller, the need for the storage device is eliminated, which makes it possible to reduce manufacturing costs of the color image forming apparatus.